1. Field of the Invention
This invention relates to radiation shields for visual display terminals (VDT) capable of being mounted in front of a VDT for attenuating, or reducing, electromagnetic E/fields emanating therefrom, such as those in the very low frequency field range from about 15,000 Hz to about 16,000 Hz, particularly in the extra low frequency range of about 3 Hz to about 3,000 Hz, that are capable of reaching the user of a VTD, said radiation shield comprising a sheet of glass carrying on at least one of the surfaces thereof a layer of conductive metal that does not appreciably impair visability through said sheet of glass and metallic grounding means ultrasonically bonded to said glass sheet solely at one location adjacent to a corner of said glass sheet.
2. Description of the Prior Art
Video display terminals (VDT) employing a cathode ray tube (CRT) on whose face is displayed information, for example, in the form of letters or numbers, are in widespread use, and it is well known that emanating therefrom are, for example, X-ray and ultraviolet radiations, very low frequency fields ranging from about 15,000 Hz to about 16,000 Hz (VLF), particularly extra low frequency fields ranging from about 3 Hz to about 3,000 Hz (ELF) pose serious health problems to users thereof. This is discussed in great detail in the literature, for example, in U.S. Pat. Nos. 4,661,856 to Schnack, 4,788,597 to Gart et al and 4,864,190 to Orr.
There have been many attempts, involving a great many approaches, to provide means to protect a VDT user from such undesirable emanations, including placing a shield between the CRT and the user. For example, Schnack in U.S. Pat. No. 4,661,856, referred to above, discloses a device adapted to be placed on the face of a CRT that includes an opaque, conductive mesh and a spherical frame having the same curvature as the CRT. Gart et al in U.S. Pat. No. 4,788,597, also referred to above, similarly disclose a filter composed of a micro mesh screen, usually made of woven fabric, comforming to the face of the CRT. In U.S. Pat. No. 4,839,736, Sugihara et al also disclose a filter designed to be attached to the face of a CRT comprising a base film having a thickness of from 10 to 250 microns, composed of an organic resin, carrying a conductive film, wherein the rigidity thereof can be improved by laminating therewith a sheet having suitable rigidity, such as other organic resins or glass. Orr in U.S. Pat. No. 4,864,190, also referred to above, also uses a screen composed of fine wires of stainless steel. Kuhlman et al in U.S. Pat. No. 4,910,090 disclose a multilayer composite filter, including a plastic substrate and an inner conductive layer.
Means other than filters designed to be mounted on or in front of a CRT have also been utilized. Thus, in U.S. Pat. No. 4,670,347 Lasik et al disclose a laminate composed of a thin polyester layer carrying a layer of silver and a second polyester layer adhesively attached to said silver coated first polyester layer adapted to surround the electronic circuitry and to be in contact with the housing. Unsworth et al in U.S. Pat. No. 4,826,718 disclose an electromagnetic shielding laminate composed of a first pane of glass carrying an electroconductive coating on its inner face, a plastic layer in contact with the inner layer, a second pane of glass over the interlayer, and a border of fine electroconductive mesh between the interlayer and the electroconductive coating in electrical contact with the electroconductive coating. In U.S. Pat. No. 4,874,903 Clarke discloses a panel composed of first pane of glass carrying a layer of silver and an electroconductive mesh in contact with the layers of silver all around the periphery of the panel.
None of the above, however, discloses or teaches the radiation shield described and claimed herein, namely, one comprising a sheet of glass carrying on at least one of the surfaces thereof a layer of conductive metal that does not appreciably impair visability through said sheet of glass and metallic grounding means ultrasonically bonded to said glass sheet solely at one location adjacent a corner thereof. The glass sheet so prepared can be placed in front of a VDT, and, when grounded will prevent any appreciable passage therethrough of electrical fields emanating from said VDT.